Two different possibilities for feeding the light beam of a laser into a scanning laser microscope are known in principle from the prior art. For example, reference may be made here to the published application of the International Patent Application WO 92/18850 and to the published application of the European Patent Application 0 592 089. Both publications describe direct feeding of the UV laser light via a mechanically rigid arrangement of optical modules such as lenses, filters, pinhole diaphragms, etc. This type of feeding gives rise to very large systems which are held together optomechanically and necessarily have long beam paths. Long beam paths lead, in turn, to instabilities of adjustment in the system. The vibrations of the laser caused by cooling represent a large problem in the adjustment of such systems. In the case of direct opto-mechanical feeding of the laser light beam into the scanning microscope, the vibrations of the laser cooling are frequently transmitted onto the microscope, thereby producing image interference and rendering permanent adjustment substantially difficult.
As an alternative to direct feeding, there is the possibility of fiber-optic feeding of light such as is described, for example, in the U.S. Pat. No. 5 161 053, for feeding UV laser light. The laser light is guided into the beam path of the microscope via a flexible optical fiber element. The laser can be quasimechanically decoupled in this way from the microscope, with the result that the vibrations of the laser caused by cooling are not transmitted onto the microscope, but instead are absorbed by the flexible optical fiber element. Furthermore, the beam lengths of the system can be substantially shortened thereby. Optical fibers for UV laser light are already on the market. However, tests have shown that their transparency drops back to less than 10% of the original transparency irreversibly after only a few hours when they are irradiated with more than 10 mW. This is presumably to be ascribed to chemical and photochemical reactions between the actual light-guiding fiber and its coating. For this reason UV lasers have up to now been fed directly into the microscope as a rule.